The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines can include spark-ignition and compression-ignition engines. Known spark-ignited engines operate by introducing a mixture of air and fuel into a combustion chamber of the engine. A piston compresses this mixture, and at predetermined crankshaft angle, a spark plug ignites the fuel and air mixture producing a flame front that propagates through the combustion chamber. The rapid increase in heat from the burned fuel triggers an increase in pressure which forces the piston downward. The use of a spark plug allows precisely timed combustion events. The compression ratio of the engine is kept to relatively low to avoid spark knock. Spark knock occurs when the fuel and air mixture auto-ignites prior to spark ignition and may cause engine damage. Consequently, spark-ignited engines are designed with geometric compression ratios between 8:1 and 11:1.
The compression-ignition engine operates at relatively high geometric compression ratios in a range of 15:1 and 22:1 and greater in particular embodiments. Higher compression ratios increase the thermal efficiency of the compression-ignited engine. The compression ignition engine operates by introducing unthrottled air into the combustion chamber, thereby increasing the efficiency by decreasing pumping losses. In a compression-ignited engine, the ignition timing is controlled by the injection of fuel into the combustion chamber near the end of the compression stroke when the trapped air within the combustion chamber is at or above an auto-ignition temperature for the fuel. The heat release of the combustion process causes an increase in in-cylinder pressure forcing the piston downward in the same manner as the spark-ignited engine.
The compression ignition engine produces emissions including particulate matter and oxides of nitrogen (NOx). Particulate matter is formed by combustion of locally rich air/fuel mixtures within the combustion chamber. These rich areas occur due to the non-homogeneity of the fuel/air charge caused by incomplete premixing of the fuel and air at ignition. Known aftertreatment devices for reducing particulate matter include particulate filters. Particulate filters trap particulate matter and are periodically purged during high temperature regeneration events.
The formation of oxides of nitrogen is a function of combustion chemistry. The compression-ignited engine produces relatively high NOx emissions in the exhaust stream after combustion of the air/fuel mixture at relatively high temperatures. Known aftertreatment systems for NOx reduction have included converter systems such as a selective catalyst reduction (SCR) device for engines operating with lean air/fuel ratios. The SCR device includes a catalyst that promotes the reaction of NOx with a reductant, such as ammonia or urea, to produce nitrogen and water. The reductants may be injected into an exhaust gas feedstream upstream of the SCR device, requiring an injection system, a reductant source and a control scheme. Additionally, engine operation may use a three-way catalyst (TWC) to produce ammonia for use as a reductant. Lower compression ratios may decrease combustion temperature thereby decreasing NOx emissions but may decrease combustion efficiency and increase engine starting difficulties at relatively cold temperatures.
One embodiment of a compression-ignition engine may include operating the engine in a premixed-charge compression-ignition (PCCI) combustion mode. The PCCI combustion mode incorporates a compression-ignition combustion system with high flow rates of cooled exhaust gas recirculation (EGR) and an early start of injection (SOI) timing. Combining a high EGR rate and an early SOI results in a long ignition delay period prior to the start of combustion (SOC). The ignition delay period exceeds the fuel injection duration during PCCI combustion resulting in a premixed combustion event at the SOC. Adequate premixing of the fuel and air, along with a high EGR flow rate, reduces the formation of locally rich regions that contribute to particulate matter formation. The high EGR rate acts as a charge diluent that suppresses the temperature of combustion below that at which significant amounts of NOx are formed.
The PCCI combustion mode is effective at relatively low engine speeds and loads where the amount of fuel injected is relatively low and the time available for premixing is relatively long. As the engine load increases, the amount of heat released in the rapid premixed burning process becomes large enough to create excessive combustion noise. This occurs even if there is sufficient premixing of the fuel and air during the ignition delay period. Excessive combustion noise is objectionable to consumers. Consequently, PCCI combustion has been limited to relatively low engine loads.